Summary: The oxygen that gives us the breath of life
is renewed by sunlight falling on plants, algae
and a particular class of bacterium called
cyanobacteria -- all of which produce mol-
ecular oxygen (O2) as a waste product of
photosynthesis (Box 1)1
. Biologists agree that
cyanobacteria invented the art of making
oxygen2
, but when and how this came about
remain uncertain.
Oxygenic photosynthesis involves about
100 proteins that are highly ordered within
the photosynthetic membranes of the cell.
Themainplayersaretwomolecularmachines,
photosystem I and photosystem II, that act as
electrochemical solar cells. With the help of
chlorophyll (the pigment that makes plants
green), they transform sunlight into electrical
current (Fig. 1). Photosystem II generates an